There has long existed a need for secure handling of confidential documents. It is well known to protect information for transmission along unprotected channels by scrambling or enciphering the information upon transmission and descrambling or deciphering the information upon receipt, using a common secret key which is known both to the transmitter and recipient.
Modern telefax machines such as the NTTFAX-43 of NTT, Japan, offer protection for confidential transmissions by locking confidential transmitted messages in the receiving machine and releasing them only in response to an appropriate access code which identifies the intended recipient.
Automatic scrambling systems for documents are described, for example, in "A confidential message handling facility for facsimile communication" by Tominaga et al, in Transactions of the Institute of Electronics and Communication Engineers of Japan, November 1982, the disclosure of which is hereby incorporated by reference. In such systems, scrambling is carried out on electrical signals bearing the confidential information. Once the document appears in a hard-copy format, it is no longer protected.
U.S. Pat. No. 4,459,611 describes a method for scrambling and descrambling of documents, by dividing their area into sub-areas and shuffling the sub-areas according to a random key. The same key is employed for reconstructing the original document in a descrambling function. In order for descrambling to take place, the scrambled document is placed on a scanner for image digitization of the scanned image.
U.S. Pat. No. 4,459,611 fails to take into account significant differences between the scrambled image as created by the scrambler and the scanned image, as the result of handling of the hard copy of the scrambled image.
These differences may include the following:
1. Dislocation and rotation of the image due to poor alignment of the scrambled document on the scanner; PA1 2. Shrinkage or expansion of the image due to humidity and temperature effects on the hard copy of the scrambled image. PA1 3. Distortion of the image due to optical errors in copying machines that may have been used to copy the scrambled image. PA1 4. Missing horizontal rows in the image due to loss of synchronization during fax transmission PA1 5. Loss of data on the scrambled image due to erasing, stains, folds, and tears that may have occurred on the hard copy. PA1 a. The required alignment accuracy is typically 1/250 of an inch, which is not normally realized in an office environment. PA1 b. No solution is provided for errors outside of the vicinity of the alignment registration marks. PA1 c. Loss of any of the registration marks may render descrambling impossible.
U.S. Pat. No. 4,459,611 suggests that alignment registration marks be employed to ensure precise alignment of the scrambled document on the scanner. This solution is not, however, practical, due, inter alia to the following reasons:
U.S. Pat. No. 4,989,244 describes a method for encrypting fax communications. The method requires that a matching key be used by both sending and receiving fax terminals in real time.
U.S. Pat. No. 5,062,136 describes a method for encrypting graphic image communication between video terminals. In this method, the image is always represented by a synchronized electronic signal and distortions like dislocation, rotation, shrinking or erasing do not occur.
U.S. Pat. No. 4,091,423 describes a scrambling system suitable for protecting facsimile communication. This method requires a real time handshake between transmitting and receiving stations and cannot be applied to ordinary fax machines without the addition of additional hardware.
U.S. Pat. No. 5,027,401 describes a method for encrypting an image into an alphanumeric (rather than graphical) representation. This method is practical for small images and to stiff and solid record material, such as ID cards.